The broad objective of this research is an understanding of how repetitions affect memory for the repeated events. The major variables under investigation are the spacing of the presentations of the repeated events and the cognitive activity used by the learner in processing the information. Increasing the spacing between the presentations of the repeated items typically produces large concommitant increases in the memorability of the items. A comprehensive theory of these spacing effects in episodic memory is proposed. The theory attempts to integrate the effects of overt repetitions with the effects of the covert processing, or rehearsal. According to the theory a repetition will enhance the memorial representation of the repeated event if any of three types of new information about the event are encoded at the repetition. The type of information encoded depends on an interaction between the physical characteristics of the repetition (e.g., spacing) and the type of rehearsal used to process the repetition. Actual performance on a memory test is ultimately controlled by the relation between the retrieval cues available at the test and the memorial representation. Manipulation of the relation produces performance that can be an increasing, decreasing, or nonmonotonic function of the spacing interval. Eight experiments are proposed to test and refine the theory. The majority of the experiments are designed to elicit one of three modes of rehearsal (maintenance, descriptive or structural), for variable amounts of time, using an incidental learning procedure. The resulting memorial representations will be analyzed using a combination of recognition, recall, and cued recall tasks. Predictions from the theory concerning performance after long spacing intervals (days to weeks) and long retention intervals, in the presence of various retrieval cues, are tested in additional experiments.